1. Field of the Invention
The present invention relates to techniques of transmitting electrical signals, and particularly to a high speed signal transmission structure that can maintain signal integrity.
2. Description of Related Art
Mutual capacitance is the coupling of two electric fields, where electrical current proportional to the rate of change of voltage in a driver flows into a target line. The shorter the distance between two electrically conductive objects, the greater their mutual capacitance. Mutual inductance is the concept that the current through one inductor can induce a voltage in another nearby inductor. It is important as the mechanism by which transformers work, but it can also cause unwanted coupling between conductors in a circuit.
Crosstalk is the electrical “noise” caused by mutual inductance and mutual capacitance between signal conductors in close proximity to each other. Crosstalk can cause digital system failure due to false signals appearing on a receiver.
In most circuits, a fast signal rise time is desirable. However, in some circuits, the fast signal rise time can cause reflections and/or electromagnetic interference (EMI) in the circuit, which can adversely affect the circuit's performance. EMI is caused by the rapid change in current as the signal changes between rising and falling states. Signal overshoot and undershoot are also undesired side effects of this rapid change in current.
FIG. 4 shows a conventional high speed signal transmission structure, and FIG. 5 shows an improved signal transmission structure conventionally configured to solve the above-described problems. Referring to FIGS. 4 and 5, a basic conventional signal transmission structure includes a driving circuit block 10, a first receiving circuit block 20, and a second receiving circuit block 30. A main transmission line 40 is connected to the driving circuit block 10, the first receiving circuit block 20 and the second receiving circuit block 30. The driving circuit block 10 includes a driving circuit 12 and a branch transmission line 14. The first receiving circuit block 20 includes a first receiving circuit 22, a branch transmission line 24, and a terminal capacitor 26. The second receiving circuit block 30 includes a second receiving circuit 32, a branch transmission line 34, and a terminal capacitor 36. The difference between the signal transmission structures of FIG. 4 and FIG. 5 is that in the structure of FIG. 5, a compensation capacitor 50 is connected to the main transmission line 40 in order to mitigate the rate of switching of signals. In the case of not having the compensation capacitor 50 (FIG. 4), the driving waveform is attenuated to very low levels due to signal reflection in transmission lines. However, in the case of having the compensation capacitor 50 (FIG. 5), the attenuation is reduced, and the rising time and the falling time are also reduced. Thereby, the switching rate is reduced.
However, employing the capacitor to depress the switching rate increases the cost of the signal transmission structure. What is needed, therefore, is a signal transmission structure, which not only depresses the switching rate and maintains signal integrity, but also can be mass-produced at a reasonable cost.